Cracking Problems and Mechanical Characteristics of PME and … · 2019-02-27 · Cracking Problems and Mechanical Characteristics of PME and BME Ceramic Capacitors Alexander Teverovsky

Cracking Problems and Mechanical Characteristics of PME and BME

Ceramic Capacitors

Alexander TeverovskyAS&D, Inc.

Work performed for Parts, Packaging, and Assembly Technologies Office,

NASA GSFC, Code [email protected]

NASA Electronic Parts and Packaging (NEPP) Program

Presented by Alexander Teverovsky at the 2018 CMSE Components for Military & Space Electronics Training & Exhibition, Los Angeles, CA, May 7-10, 2018.

List of Acronyms


2

BME base metal electrode IFT Indentation Fracture Test

C-SAM C-mode scanning acoustic microscopy IR insulation resistance

DCL direct current leakage IWT ice water test

DF dissipation factor MLCC multilayer ceramic capacitor

ECM electrochemical migration MOR modulus of rupture

EDS energy dispersive spectroscopy PME precious metal electrode

EM electrical measurements RH relative humidity

ESR Equivalent series resistance TSD terminal solder dip

FA failure analysis VH Vickers hardness

Abstract


3

Most failures in MLCCs are caused by cracking that create shorts betweenopposite electrodes of the parts. A use of manual soldering makes this problemespecially serious for space industry. Experience shows that different lots ofceramic capacitors might have different susceptibility to cracking under manualsoldering conditions. This simulates a search of techniques that would allowrevealing capacitors that are most robust to soldering-induced stresses.Currently, base metal electrode (BME) capacitors are introduced to high-reliabilityapplications as a replacement of precious metal electrode (PME) parts.Understanding the difference in the susceptibility to cracking between PME andBME capacitors would facilitate this process.This presentation gives a review of mechanical characteristics measured in-situon MLCCs that includes flexural strength, Vickers hardness, indentation fracturetoughness, and the board flex testing and compare characteristics of BME andPME capacitors. A history case related to cracking in PME capacitors thatcaused flight system malfunctions and mechanisms of failure are considered.Possible qualification tests that would allow evaluation of the resistance ofMLCCs to manual soldering are suggested and perspectives related tointroduction of BME capacitors discussed.

Outline

Problems with cracking of MLCCs. In-situ mechanical testing.

Flexural strength. Vickers hardness. Indentation fracture toughness. Board flex testing.

Failure history case. What can be done to mitigate manual soldering

cracking. Conclusion.


4

Cracking-Related Problems Mechanism of cracking during manual soldering. Revealing cracks in capacitors (loose and soldered).

Electrical measurements (C, DF, IR, VBR) Electromechanical effects Visual, radiography, ultrasonic analysis

Effects of cracking on reliability in humid and HT environments. Robustness of MLCCs towards thermo-mechanical stresses.

IWT (ice water testing) TSD (terminal solder dip testing)

In-situ mechanical testing. Flexural strength Vickers hardness Indentation Fracture Test Board flex testing

Susceptibility to cracking of PME and BME capacitors.


5

Do different types of MLCCs have different susceptibility to cracking under manual soldering conditions and how it can be revealed?

Degradation of MLCCs with cracks and the effectiveness of different techniques is described in various publications and reports posted at the NEPP web site.

Flexural Strength

6Presented by Alexander Teverovsky at the 2018 CMSE Components for Military & Space Electronics Training & Exhibition, Los Angeles, CA, May 7-10, 2018.

The test is described in AEC-Q200, but acceptance criteria are up to the users.

223bdFLMOR =

AEC-Q200-003

CDR35BX104AKUS, 0.1uF, 50V, size 1825, Mfr.C

MOR, MPa

cumu

lative

pro

babil

ity, %

40 4001001

5

10

50

90

99

initialactivated rosin fluxpolishedmoisture: 160hr, 85% RH, 22Cmoisture: 160hr, 85% RH, 22CTSD350 & moisture

10N5N 2.5N

No effect of possible flaws but surface cracks reduce MOR. Smaller size MLCCs have greater strength – Benefits of BMEs. No substantial difference between BME and PME capacitors. Variations of MOR values from lot to lot might exceed 50%. The test can be used for relatively large (≥1206) parts. Same size capacitors can be used for comparative analysis.

Modulus of rupture 4

7

4 7 19 13

58

8

0

100

200

300

400

500

600

1206 1210 1808 1812 1825 2225

char

acte

ristic

MO

R, M

Pa

case size

Effect of case size on MOR

PME

BME

3

Vickers Hardness


In-situ VH measurements are possible using MLCCs with relatively thick cover plates. P should be low so the depth of the indentation is < 2x the thickness of the cover plate.

No correlation between strength and VH for ceramic materials. No significant difference between PME and BME capacitors. Reduction of errors might allow for revealing differences in lots.

Hardness is a resistance to indentation.Testing is specified in ASTM C1327-15 (2015)

2854.1

DPVH ×

=

Vickers Hardness Test for PME and BME Capacitors

VH, GPa

cumu

lative

pro

babil

ity, %

7 128 9 10 111

5

10

50

99

BME

PME

90% confidence

y = 191.65x

0

100

200

300

400

500

600

700

0 1 2 3 4

D^2

, um

^2

load, N

Vickers test, LT capacitors

SN1

SN2

SN3

SN4

SN5

Indentation Fracture Test


Test results depend on environments and time of exposure. IFT might be useful for selecting parts for manual soldering, but

more work is necessary to reduce errors and select criteria. No significant difference between PME and BME capacitors.

Fracture Toughness: the ability of a material to withstand stresses in the presence of cracks.

IFT technique is the most controversial.

= 5.1

5.0

_ cP

VHEIFT MRξ

PME BX

PME BP

BME X7R

IFTavr 1.08 1.46 0.95STD 0.20 0.19 0.23

N 12 2 60

20

40

60

80

100

120

140

160

180

0 1 2 3 4 5

c^1.

5, (u

m)^

1.5

load, N

PME_L capacitors

PME_L1, 1.7

PME-L2, 1.32

PME_L3, 1.49

PME_L4, 1.20

1.03 1.07

0.83

1.35

1.000.89

1.33

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

Kc,

Mpa

_m^0

.5

Different lots of capacitors

L1

L2

L3

Board Flex Testing


AEC-Q200-005 specifies conditions and the size of the test board. “A failure is when a part cracks or causes a change in the parametric being monitored.”

M32535 allows for multi-chip boards. Failure criteria: C >+/-10% at δ = 2 mm.

Factors affecting test results: Orientation of the component; Attachment with Ag-epoxy absorbs stress; Solder fillet height, and thickness under the chip; Solder type (less cracking for Pb-free alloys) MLCC material (X7R weaker than COG) Larger chips experience greater stress and have

greater susceptibility to cracking. This test is widely used to address cracking during de-panelization. Results are affected by variety of factors. Conditions for using multi-chip boards need additional analysis.

Board Flex Testing, Cont’d


Mechanical stresses affect characteristics of class II capacitors. Deviations of ε can change C during flex testing up to a few %. Variations of C might be reversible even in the presence of cracks.

0

1

2

3

4

5

6

00.05

0.10.15

0.20.25

0.30.35

0.40.45

0.5

0 50 100 150 200

defo

rmat

ion,

mm

C, u

F

time, sec

PME 2225 X7R 0.47uF 50V A2 SN1

C, uF

bend, mm

0

1

2

3

4

5

6

9.40E-2

9.45E-2

9.50E-2

9.55E-2

9.60E-2

9.65E-2

0 50 100 150 200

defo

rmat

ion,

mm

C, u

F

time, sec

BME 0603 X7R 0.1uF 50V SN2

C, uF

bent, mm

0

0.5

1

1.5

2

2.5

3

3.5

10.4

10.5

10.6

10.7

10.8

10.9

11

11.1

0 50 100 150

defo

rmat

ion,

mm

C, n

F

time, sec

PME 0805 X7R 0.01uF 25V SN1

C, uFbend, mm

0

0.5

1

1.5

2

2.5

3

3.5

2.22.222.242.262.282.3

2.322.342.362.382.4

0 50 100 150

defo

rmat

ion,

mm

DF,

%

time, sec

PME 0805 X7R 0.01uF 25V SN1

DF, %

bend, mm

0

0.5

1

1.5

2

2.5

3

3.5

-1.0E-09

-5.0E-10

0.0E+00

5.0E-10

1.0E-09

1.5E-09

2.0E-09

2.5E-09

3.0E-09

0 100 200 300

defo

rmat

ion,

mm

curr

ent,

A

time, sec

PME 0805 X7R 0.01uF 25V SN1

I, A

bend, mm

Board Flex Testing, Cont’d


Variations of capacitance having cracks with load are reversible.Degradation of C by steps indicates partial separation of electrodes.Distributions of δcr (0.9Cinit) might be more effective for assessment

of the vulnerability to cracking compared to 2 mm pass-fail criteria.No substantial difference in the flex cracking between PME and

BME capacitors.A smaller size of BME compared to similar value PME capacitors

makes BME less vulnerable to flex cracking.

0

1

2

3

4

5

6

0

0.002

0.004

0.006

0.008

0.01

0.012

0 60 120 180 240de

form

atio

n, m

m

C, u

F

time, sec

PME 0603 X7R 0.01uF 25V SN2

C, uF

bend, mm

0

1

2

3

4

5

6

7

0.05

0.06

0.07

0.08

0.09

0.1

0.11

0 50 100 150 200 250

defo

rmat

ion,

mm

C, u

F

time, sec

BME 0603 X7R 0.1uF 50V SN1

C, uF

bend, mm

A History Case


On-orbit anomalies after months of operation were attributed to excessive leakage currents in CDR35 capacitors.

The parts were soldered manually and suspected of having cracks. Testing of a spare unit on the ground also showed increasing

leakage currents after several weeks of operation. FA: the failure was due to delaminations and cracking in the part. No external cracks on the failed lot were observed. Acoustic microscopy showed that a substantial proportion of parts

had delaminations at the termination areas.

Courtesy of L.Panashchenko and R.Weachock

A History Case. Test Plan.


To check whether the flight lot has high susceptibility to cracking and to identify techniques for assessment of the robustness of parts to manual soldering, 20 capacitors from the flight lot (lot A) and a reference lot (lot C) were tested in parallel.

o Before stress testing the parts were characterized by mechanical, electrical and acoustic (C-SAM) tests.o Terminal solder dip testing (TSD350) was used to simulate manual soldering stresses.o Leakage currents were monitored with time at different voltages and environmental conditions.

A History Case. Initial Characteristics.


Some difference between lots was revealed by acoustic microscopy and flexural strength testing.

CDR35 0.47uF 50V capacitors

MOR, MPa

cumu

lative

pro

babil

ity, %

50 2001001

5

10

50

90

99

Mfr.A

Mfr.C

(113/8.6 MPa)

(148/17.7 MPa)

Optical examination: no anomalies.EDS: similar composition of ceramics.C_SAM: no defects in the lot C and

corner delaminations in lot A.No substantial difference in distributions

of C, DF and IR between the lots.Both lots had DCL at 2VR <10-10 A.VH and IFT did not reveal significant

difference. However, the flexural strength was greater for lot C.

y = 2E-07x-1.071

y = 1E-07x-1.041

1.E-10

1.E-9

1.E-8

1.E-7

1.E-6

1.E+0 1.E+1 1.E+2 1.E+3

curr

ent,

A

time, sec

CWR35 0.47uF 50V Mfr.A

50V

0_50V

100V

0_100V

courtesy of C.Greenwell

A History Case. Effect of TSD350.


20 samples were subjected to 3 cycles of TSD at 350 ºC

Two lots had a substantially different propensity to cracking and electrical failures after manual soldering simulations.

visual C-SAM EM 1000hr at 50V, 22ºC/85%RH

Lot A 20/20 18/20 1/20 6/20Lot C 1/20 0/20 0/20 0/20

0.E+0

2.E-9

4.E-9

6.E-9

8.E-9

1.E-8

0 200 400 600 800 1000

curr

ent,

A

time, hr

Mfr.C after TSD350 and CSAM at 22C 85% RH, 50V

1.E-10

1.E-9

1.E-8

1.E-7

1.E-6

1.E-5

1.E-4

0 200 400 600 800 1000

curr

ent,

A

time, hr

Mfr.A after TSD350 and CSAM at 22C 85% RH, 50V

SN A1 SN A5SN A6 SN A9SN A15 SN A18SN A20 1 G_uF

Lot CLot ALot A

A History Case. Failure Analysis.


Cracks in lot A started from the surface and continued as metal/ceramic delamination.

To evaluate interaction of cracks with delaminations, virgin and post-TSD samples were fractured in the middle.

No delaminations on virgin samples from lot A and on both virgin and post-TSD samples from lot C.

TSD testing and fracturing resulted in delaminations located mostly at electrodes close to the surface of capacitors.

Fractured post-TSD samples

A History Case. Failure Mechanism.


Cracking and delaminations caused by electroplating occur more often with PME than with BME capacitors.

Probability of failures after manual soldering was increased due to the presence of delaminations.

Corner location indicates that delaminations might be due to formation of terminals.

Reasons for corner delaminations [*]: Thick Ni layers increase mechanical stresses.Generation of H2 during electroplating:• Decreases fracture toughness of ceramics;• Removes PdO barrier on Ag/Pd electrodes,

weakens the interface and facilitates ECM of Ag.• Fast evolution of H2 might cause pop-corning

during soldering.

electroplating

formed capacitor

soldering

[*] "Susceptibility to Cracking of Different Lots of CDR35 Capacitors,", NEPP report2017, https://nepp.nasa.gov/files/29210/NEPP-TR-2018-Teverovsky-CDR35-Capacitors-TN52049.pdf

https://nepp.nasa.gov/files/29210/NEPP-TR-2018-Teverovsky-CDR35-Capacitors-TN52049.pdf

Resistance to Manual Soldering Test


A combination of TSD350, C-SAM, EM, and humidity tests can be used to select parts more robust to manual soldering.

Depending on criticality some tests can be skipped or replaced.

Initial EM (C, DF, IR)• No failures allowed.• In case of failures →

reject or rescreen.

TSD350, 2 sides 3 cycles• If > 25% samples

have visual cracks → reject.

EM (C, DF, IR)• No failures allowed.• Analyze distributions

for outliers.

C-SAM• If a sample has >25%

delaminated area→ reject.• if > 25% samples have

corner cracks → reject.

Monitoring I-t at 85%RH, VR for 1000hr• If current spikes

exceed Icr → reject.

None of the tests provides reliable information regarding the susceptibility to cracking, but some test have better sensitivity.

Example of test sequence

Conclusions


In-situ mechanical testing of MLCCs, and the flexural strength in particular, can reveal lot-to-lot variations.

Different lots of MLCCs do have different susceptibility to cracking and failures caused by manual soldering.

No substantial difference between mechanical characteristics of PME and BME capacitors.

Capacitors with cracks can pass ground phase testing, but fail during the mission.

Combination of TSD350, C-SAM and electrical testing can be used to mitigate the risk of failures.

Due to a smaller size and different degradation mechanisms, BME capacitors have a lesser probability of failures caused by manual soldering compared to PME parts. However, more problems might be expected with small (≤0603) size MLCCs.

Documents

Cracking Problems and Mechanical Characteristics of PME and … · 2019-02-27 · Cracking Problems and Mechanical Characteristics of PME and BME Ceramic Capacitors Alexander Teverovsky